1. Field of the Invention
This invention relates generally to apparatus and methods in which surface-based sensors measure incident charged-particle currents, charging voltages, temperatures and other physical parameters at a work piece surface during plasma processing, and more particularly to a semiconductor wafer utilizing surface-based sensors to provide real time measurement of plasma characteristics adjacent to the wafer surface as well as select physical properties during plasma processing.
2. Brief Description of the Prior Art
Spatial and temporal variation in plasma characteristics and the work piece surface temperature can strongly influence the performance and yield of plasma-based processes, such as those encountered in semiconductor manufacture. In such processes, variations in physical plasma parameters that occur adjacent to the process work piece directly impact process metrics which may include the following: (1) etch rates and etch profile control, (2) surface charging effects and device or film damage, and (3) thin film deposition rates, density, coverage, morphology, stress and adhesion. Some common plasma parameters that drive surface processes on a work piece, such as a semiconductor substrate wafer, include charged-particle density and flux (ion and electron density), apparent electron temperature, ion energies, neutral gas temperature, density and flux of reactive gas species, and plasma radiative emissions. It is also known that surface temperature of the work piece or wafer can play a very critical role in many of the surface reactions and results of the plasma process.
Because of the criticality of both plasma characteristics and substrate temperature and their impact on process yield, several workers have attempted to monitor plasma characteristics and surface temperatures during processes by means of diagnostic probes that are directly mounted to a work piece, such as a semiconductor wafer substrate. In these devices, diagnostic probes such as thermocouples, DC-biased electrical probes, ion energy analyzers, and surface charging collectors have been used to measure spatial and temporal variation of surface temperature, selected plasma parameters, and plasma-induced charging effects. One such device is the Stanford Plasma On-wafer Real Time (SPORT) probe as described in an article by S. Ma and J. P. McVittie in the proceedings of the 1996 International Symposium on Plasma Process-Induced Damage pg. 20-23. The SPORT probe is capable of measuring electrostatic charging and plasma-induced currents at the wafer surface. The SPORT probe utilizes large conductive pads placed on a thick oxide layer of a silicon wafer. Polysilicon leads make direct current contact to the pads and the silicon substrate. Wire leads connected to the edge of the wafer carry current and voltage signals outside the plasma-processing chamber to a low pass RF filter to a dc measurement circuit. By means of the external measurement circuit, plasma induced charging voltages are measured between the pads and the substrate in order to quantify plasma induced electrostatic charging effects that could result in damage to electrically sensitive semiconductor device structures during plasma processing and fabrication.
Another apparatus is described in U.S. Pat. No. 5,801,386 issued to Valentin N. Todorov et al. This patent discloses an apparatus that comprises a plurality of conductive collector pads for detection of plasma induced ion currents and self-biased voltages. The collector pads are arranged in an array so that plasma-induced properties of ion current and self-bias voltage can be spatially resolved over the wafer surface in real time. Each collector pad is connected to a conductive lead that extends outside the chamber to an external data acquisition system.
Also in U.S. Pat. No. 5,959,309 entitled xe2x80x9cSensor to Monitor Plasma Induced Charging Damagexe2x80x9d, Tsui, et al. describe a discrete monitoring circuit that measures the plasma-induced voltage and currents to a sampling pad or antenna that is in communication with a ground or common. In this device, the sampling pad is connected to ground through a blocking diode, a blocking transistor, and a storage capacitor. Once the monitor is exposed to the plasma, the voltage between the charged pad or antenna and the electrical common or ground is recorded by charging a storage capacitor. The workers also disclosed how a plurality of these monitors, each with different loading resistances, can be integrated onto a single chip to measure the magnitude of the charging voltage and the plasma-induced current between the antenna and common or ground of the chip. The charging voltage and pad-to-common currents are determined by electrically measuring the voltages of the storage capacitors after the sensor or chip is removed from the plasma processing environment.
Freed et al. describe the development of sensor methods in xe2x80x9cAutonomous On-Wafer Sensors for Process Modeling, Diagnostic and Controlxe2x80x9d (IEEE Transactions on Semiconductor Manufacturing, Vol. 14, No. 3, pp 225-264). This paper describes the basic design challenges faced in the development of an in situ or in-line wafer sensor including power source concepts, wireless communications methods, and electrical isolation of on-wafer electronics. In their examples, they illustrate two design concepts. In the first design concept is an on-wafer thermistor sensor powered with re-chargeable batteries and voltage regulator. The design also includes an A/D converter and LED optical communication electronics for transferring data off the wafer in a thermally elevated process environment and a plasma etching environment. In another version of the design, the workers illustrate how a van der Pauw sheet resistance device may be adapted with CMOS processing methods for measuring polysilicon etch rates. They demonstrate the viability of this sensor with a wired wafer as applied to a XeF2 (non-plasma) etching reactor. These devices have varying degrees of effectiveness in monitoring the wafer temperature or the characteristics of a plasma body adjacent to the wafer when disposed in a plasma processing environments. However, all the examples of the prior art have several limitations that restrict their use for obtaining real-time plasma and substrate temperature measurement within a plasma processing system. Many of these measuring devices are intrusive in that they require the use of wires into the plasma processing system and others are passive recording devices that cannot make real-time measurements. Also, those devices that do not use external wires are limited in on-time operation and power supply current draw since they rely entirely upon on-board battery power sources that have limited milliamp-per-hour capacity or limited sustainable trickle current capacity when attempting to power a sizable array of sensors, microprocessor(s) and wireless communication subsystems. Moreover, in the context of these in situ measurement apparatuses, none of the prior art teachings discuss in detail how to devise a sensor capable of obtaining plasma measurements, such as charged-particle (ion or electron) fluxes, densities and energies that can be adapted to a wireless sensing apparatus.
It would be desirable if there were provided a surface-based sensor apparatus that could make spatially resolved, real-time measurements of plasma properties adjacent to the surface of the apparatus, as well as other properties such as surface temperature. It would also be desirable if the device were non-invasive to the plasma process and if the time-dynamic data recorded by the device could be either transmitted in real-time through a wireless interface or, alternatively, be recorded for downloading once the sensor apparatus is removed from the plasma process chamber. It would be further desirable if the device had a self-contained power supply means that did not rely entirely upon the limited lifetime or trickle current ratings of a battery or alternative conventionally power source.
There is provided by this invention an apparatus for making real time measurements of incident plasma currents, charging surface voltages, and other plasma related parameters as well as surface temperatures within a plasma processing environment. The apparatus is generally comprised of at least one integrated sensor circuit mounted on a work piece such as a silicon wafer substrate. The sensor is comprised of either a dual floating probe to measure ion currents from the plasma, a topographical dependent charging structure to measure plasma induced surface charging effects, filtered photodiodes to measure optical emissions signals, a thermal sensing device to monitor surface temperature or a combination thereof The sensor inputs are transmitted to a central microprocessor and transceiver that is provided for processing sensor signals, memory storage, and real-time transmission of data via infrared- or rf-wireless communication to a receiver outside the plasma chamber. To power the apparatus, a battery is contained within the apparatus to provide power to the integrated sensor devices, microprocessor and wireless transceiver. Alternately, the apparatus may include one or more topographically dependent charging structures to electrostatically couple power from the plasma boundary that is then regulated and used to provide all or part of the power to the apparatus electronics. The apparatus is particularly useful in spatial and real-time monitoring of plasma and substrate conditions in plasma-based non-depositing processes such as etching, photo-resist stripping or surface cleaning, but could be applied to some plasma-based deposition processes with the appropriate configuration or adaptation of the integrated sensing devices.